Copper-aluminum heat exchangers may comprise aluminum plate-fins stacked in parallel with a plurality of aluminum hair-pin tubing sections going through the plate-fins. The aluminum heat exchanger is connected to copper inlet and outlet tubes. Typically, the aluminum heat exchanger is connected to copper inlet and outlet tubes by connector or extension tubes. In the heat exchanger industry, these are also called stub-out tubes. The copper inlet and outlet tubes carry the refrigerant to and from the heat exchanger.
Plate-fin and tube heat exchangers are used in a wide variety of applications including, but not limited to, air conditioning and refrigeration where it is desired to exchange heat between two fluids, usually a pure liquid or a liquid undergoing a phase change to or from a gas, flowing in the heat exchanger tubes and gas, usually air, flowing around the heat exchanger plate-fins and tube exteriors.
In such a heat exchanger, a plurality of thin plate-fins are arranged parallel to each other between two tube sheets. Heat exchanger tubes pass through holes in the tube sheets and plate-fins. There is a firm fit between the tubes and the plate-fins so that the effective surface area, and thus the heat transfer area, of the heat exchanger tubes is increased by the area of the plate fins. Because of this increase in surface area, a plate fin and tube heat exchanger offers improved heat transfer performance over a plain tube type heat exchanger of the same size.
A common method of manufacturing this type of heat exchanger is to first assemble a plurality of plate-fins between two tube sheets, then lace a plurality of hair pin tubes through selected holes in the plate-fins and similar holes in each of the tube sheets. Next, bells are formed in the end of hairpin tubes, then the legs of the tubes are expanded to insure a tight mechanical fit between the tubes and plate-fins.
Currently there are difficulties in joining aluminum tubes to copper inlet and outlet tubes. In order for the heat exchanger to function properly, the joint between the aluminum connector and the copper inlet tube must hold refrigerant at high operating pressures. Furthermore, the joint must be capable of holding for many years while operating at pressures that are multiplied by a safety factor. Additionally, the joint must also hold these pressures in an environment that subjects the joint to large amounts of cyclic stresses resulting from mechanical and thermal loading.
In response to these challenges, manufacturers of heat exchangers have developed mechanical joints and other methods in an attempt to provide heat exchangers with a tight fit that is suitable for long-term use. There are several methods for joining the aluminum tube to the copper tube. For example, joining the two tubes can be done by soldering or brazing. To date, these methods have proven to be inadequate.
Another problem with copper-to-aluminum joints is the tendency for galvanic corrosion. This problem is magnified if the brazing alloy used to braze the aluminum and copper does not completely fill the joint. This is a common problem in brazing joints.
Galvanic corrosion occurs when two dissimilar metals make contact with one another in the presence of an electrolyte, such as water, thereby forming a galvanic couple. The mere presence of an electrolyte will allow for galvanic corrosion to occur when the base materials are in close proximity, even when they are not in direct contact. The more noble metal (i.e., the more cathodic on the galvanic series) provides additional surface area for a reduction reaction to occur on. This reaction accelerates the oxidation/corrosion of the less noble metal (more anodic on the galvanic series). The extent of the corrosion is greatest at the interface of the two metals, but may also occur at some distance away from the actual interface.
Heat exchangers made of copper tubes and aluminum fins have a corrosion durability that is about ten times lower than heat exchangers made with aluminum fins and aluminum tubes. As explained above, when two dissimilar metals are in contact with each other in a corrosive environment, a galvanic couple forms and the more noble metal promotes the corrosion of the more active metal. In the case of heat exchangers having copper tubes and aluminum fins, the copper is the more noble metal, while the aluminum is more active.
The adverse role of the more noble metal, copper, is that: (1) it provides additional surface area for the reduction reaction of the corrosion process to occur on, and (2) corrosion reduction rates are very high on copper. These two factors accelerate the corrosion process, resulting in a ten-fold increase in the corrosion rate of the aluminum fins.
There are several methods for preventing galvanic corrosion. One method involves coating the exposed surface of the metal with various types of paint. Another method of protecting the heat exchanger core involves coating it with a flux containing zinc chloride (ZnCl2). Yet another method involves aluminizing the copper. All of the above methods to prevent galvanic corrosion have provided limited success.